1. Field of the Invention
The present invention relates to an apparatus and methods for measuring the shape of both sides of a plate, and more particularly, for measuring a surface and a back surface of a plate when the shape of each of the surfaces of the plate is important such as for photomasks made of quartz glass for use in manufacturing of high-precision, large-sized liquid crystal displays.
2. Related Art
For example, in the manufacturing of a TFT (thin film transistor) array of a liquid crystal display plate, a mask pattern formed by a shielding film on a surface of a photomask is exposed and projected by a photolithographic technique such that the pattern is transferred onto a mother glass. Then, the TFT array is formed on the mother glass by the photolithographic technique and a so-called process technique. Similarly, a color filter of a liquid crystal display plate is else manufactured by a lithographic method referred to as a dye impregnating method. The manufacturing of both the TFT array side and the color filter side typically requires the use of a large-sized photomask. In order to transfer the mask pattern with high precision, the large-sized photomask is generally made of a synthetic quartz glass having a small coefficient of linear expansion.
The demand for larger-sized mother glass used for manufacturing the liquid crystal display plates has been increasing. Currently, large-sized photomasks having dimensions of 1500 mm×1500 mm are being used. The plate thickness for such large-sized photomasks is 10 mm to 20 mm.
In these large-sized photomasks, the degree of flatness over the surface of the photomask on which the mask pattern is formed is important. For each photomask, the degree of flatness of a surface is measured, and a strict quality management is used for selecting photomasks within a standard range. Therefore, various apparatuses for measuring the degree of flatness of the surface of large-sized photomasks have been proposed so far (for example, Japanese Patent Application, Publication No. H03-90805 and 2000-55641). Some of these apparatuses have already been used practically for managing the degree of flatness of large-size plates such as large-sized photomask plates.
For liquid crystal display plates, resolution has also been increasing from, for example, VGA to SVGA, XGA, SXGA, UXGA and QXGA. Furthermore, a method has been used for making TFTs that uses low-temperature polysilicon and forming an IC for a driver in an outer peripheral portion of the mother glass separately from a pixel of the display. As a result, demand has increased for improvements in precision in pattern transfer on the TFT array side, particularly in the precision of the exposure and projection alignment of the pattern.
To improve the pattern transfer precision in photolithography has required the use of strict quality management controls on the degree of flatness over the entire surface of the photomask and the degree of flatness of of a back surface of the photomask which is opposed thereto. This will be described with reference to FIG. 8. In this configuration, FIG. 8A shows the case in which a surface 102 of a photomask 101 takes a convex shape and a back surface 103 takes a relatively flat surface. FIG. 8B shows the case in which the surface 102 of the photomask 101 takes a relatively flat shape and the back surface 103 takes a concave shape.
As shown in FIG. 5A, when the surface of the photomask 101 takes the convex shape, an exposed light 104 shown in a solid line which is incident from the back surface 103 side of the photomask 101 causes winding due to the convex shape of the surface 102, thereby exposing a photosensitive film (not shown) provided on a surface of a mother glass 105. Due to the winding of the exposed light 104, a shift is generated on a transfer position in the pattern transfer. A dotted line in the drawing shows an ideal optical path in which the surface 102 does not have a convex shape and through which the light advances in a straight direction. By increasing the degree of flatness of the surface of the photomask to approximate that of a surface that would produce an ideal optical path, the precision in the pattern transfer on the TFT array side would be enhanced.
Also in FIG. 5B, the same situation as that in FIG. 8A is generated though the extent of the effect on the incident light is smaller. As shown in FIG. 5B, when the exposed light 104 is incident from the back surface 103 side having a concave shape, the optical path reaches the surface 102 with winding thereon. The degree of the winding is reduced by the surface 102. Accordingly, the photosensitive film (not shown) provided on the surface of the mother glass 105 is exposed. In this case, a shift in transfer position is also produced during the pattern transfer, even if it is smaller than that of FIG. 8A.
If improvements in the resolution of the liquid crystal display or in the precision of the pattern transfer to embed an IC for a driver are desirable, the positional shift of the pattern transfer caused by the concavo-convex shape of the back surface of the photomask is a drawback. For this reason, there exists a need to increase the degree of flatness of the back surface of the photomask to approximate that of a surface that would produce an ideal optical path and to use strict quality management to control the degree of flatness.
However, there has not been developed an apparatus for measuring shapes of both sides which can measure the shapes of a surface and a back surface of the photomask for a large-sized liquid crystal with high precision at the same time or a method of easily measuring the shapes of both sides or the degree of flatness of both sides with high precision.